One of the most important metallurgical operations for the production of steel is controlled deoxidation in the melt, since the presence of excessive free oxygen will result in porosity of the finished steel. Although a large number of different deoxidizing agents may be used in the process of steelmaking, aluminum has become the most widely used deoxidizer in the production of cast steels, and addition of aluminum at some stage of the steelmaking process is almost universal.
Very effective deoxidation may be achieved with only small additions of aluminum to the melt, but it is well understood in the steelmaking industry that control of the amount of aluminum introduced into the melt in relation to the amount of molten steel is important in controlling the presence of insoluble inclusions in the finished steel. Small additions of aluminum may result in the formation of inclusions resulting in poor steel ductility, whereas excessive additions of aluminum may result in the presence of aluminum nitride precipitates which increase the brittleness of the finished steel.
In addition, although only relatively small amounts of aluminum are required for effective deoxidation of the melt, increasing costs of metallic aluminum have caused steelmakers to become more concerned with the cost of aluminum addition during the steelmaking process. This consideration has lead to more careful control of the total amount of aluminum added to the melt, and steelmakers have become increasingly concerned with the efficiency of the aluminum or aluminum based deoxidizing agent used.
Traditionally, essentially pure metallic aluminum has been the most common form of aluminum deoxidizing agent used in the steelmaking process, in any convenient form such as notch bars, small ingots, shot, and chopped wire. The use of essentially pure aluminum presents some significant disadvantages, however, arising primarily from its low density as compared to the molten steel to which the aluminum is added. The density of liquid aluminum at steelmaking temperatures of approximately 1600 degrees C. is only about 2 grams per cubic centimeter, whereas the density of molten steel at the same temperature is greater than 7 grams per cubic centimeter. Therefore, when aluminum is added to the melt, it will float at the steel/slag interface, where the aluminum rapidly oxidizes, with relatively small amounts of the aluminum actually making contact with the molten steel. The efficiency of the aluminum as a deoxidizing agent is thus limited by the rate at which oxygen in the melt can diffuse upward to the slag/steel interface, and deoxidation performance is erratic.
In an effort to overcome this disadvantage of the use of essentially pure metallic aluminum, steelmakers have sought other forms of aluminum deoxidizing agents. One such agent is an iron/aluminum alloy mixture with aluminum, known in the industry as ferroaluminum. The composition of ferroaluminum is nominally twenty-five to forty percent aluminum and seventy-five to sixty percent iron or steel. The solubility of aluminum is about twelve percent, so the ferroaluminum agent consists of a combination of aluminum/iron alloy to the solubility concentration, with the remainder of the aluminum existing as pockets of essentially pure aluminum agglomerated with the alloy material. The density of ferroaluminum is about twice the density of pure aluminum, resulting in deeper penetration of the ferroaluminum oxidizing agent into the molten steel. Because of the deeper penetration, resulting in improved contact between the deoxidizing agent and the molten steel, ferroaluminum does produce an improved deoxidation efficiency in comparison to aluminum alone, but still suffers from certain disadvantages.
It has been found that adhesion between the aluminum/iron alloy and the pockets of metallic aluminum present in the ferroaluminum agent is poor, and the ferroaluminum product displays a pronounced tendency to decrepitate, with crumbling and separation, during storage and handling prior to its addition to the melt. It has also been found that ferroaluminum displays the same tendency to separate into discrete pieces of metallic aluminum and aluminum/iron alloy as a result of thermal shock upon its introduction into molten steel, even if its physical integrity has been maintained prior to that introduction. With such separation, much of the theoretical efficiency of the ferroaluminum agent is lost in practical use of the agent, though its deoxidation efficiency is still higher than that of essentially pure aluminum alone. Further, iron is an ineffective deoxidizing agent, and the physical mixture of iron with aluminum in the alloyed portion of the ferroaluminum agent dilutes the total amount of aluminum available for reaction per unit of surface area as compared to pure aluminum, resulting in lower relative reactivity. Thus the ferroaluminum deoxidizing agent represents a compromise between density and reactivity.
There remains a need, therefore, for a method of aluminum deoxidization of a ferrous melt which utilizes an aluminum deoxidizing agent with an apparent density greater than that of pure aluminum, for improved penetration into the melt, without a sacrifice of reactivity of the agent as compared to pure aluminum.